1. Place the tomatillos in a saucepan, fill with water and bring to a simmer. Simmer 10 minutes. Drain and place in a blender. Add the chiles, chopped onion, garlic, salt and cilantro. Blend until smooth.2. Heat the oil in a large, heavy saucepan or skillet over medium-high heat. Drizzle in a drop of the tomatillo purée to test the heat. If it sizzles and sputters immediately, the oil is hot enough. Add the tomatillo purée, and stir constantly until it thickens and begins to stick to the pan, about 5 minutes. When you run your spoon down the middle of the pan it should leave a canal. Stir in the stock, bring to a simmer, and simmer 15 to 20 minutes, stirring often, until it has reduced by about a quarter and coats the front and the back of a spoon like cream. Keep warm.3. Preheat the oven to 300 degrees. Line a sheet pan with foil and oil the foil. Season the fish fillets with salt and pepper and lay on the foil. Place a pan of just boiled water on the floor of your oven and place the baking sheet with the fish in the oven on the middle rack. Bake 10 to 20 minutes, depending on the thickness of the fillets, until the fish is opaque on the surface and you can pull it apart with a fork. Remove from the heat, transfer to plates or a platter, and spoon on the sauce. Garnish with cilantro and lemon or lime wedges, and serve. You will have some sauce left over.Yield: Serves 4Advance preparation: The salsa will keep for 3 or 4 days in the refrigerator. Reheat gently.Nutritional information per serving: 158 calories; 4 grams fat; 0 grams saturated fat; 2 grams polyunsaturated fat; 1 gram monounsaturated fat; 67 milligrams cholesterol; 6 grams carbohydrates; 2 grams dietary fiber; 443 milligrams sodium (does not include salt to taste); 23 grams protein